A communication device such as a cellular phone requests dial-up connection to a server on the Internet to communicate with the server by being assigned an internet protocol (IP) address. A user of the communication device thereby is able to surf homepages prepared in servers on the Internet. Incidentally, the communication device having a dynamic IP address can communicate through an IP with another communication device having a static IP address by identifying the communication device based on the static IP address, which has been previously assigned.
The communication device having the dynamic IP address cannot contrarily communicate through the IP with another communication device having a dynamic IP address owning to the difficulty of identifying the communication device based on the dynamic IP address, which has been temporarily chosen and assigned by the server.
In a system including cellular phones as the communication devices, as shown in FIG. 4, dynamic IP addresses X and Y (for instance, 32 bit or 128 bit figures) are assigned to cellular phones 1 and 2, respectively. Here, the cellular phone 1, which has the dynamic IP address X, cannot identify the cellular phone 2 due to impossibility of identifying the unknown dynamic IP address Y. As a result, it is impossible to perform communication through the IP with the cellular phone 2.